Semiconductor device

ABSTRACT

A semiconductor device, including a semiconductor layer of a first conductivity type, having a main surface with a diode trench formed therein, an inner wall insulating film, including a side wall insulating film, formed along side walls of the diode trench, and a bottom wall insulating film, formed along a bottom wall of the diode trench and having a thickness greater than a thickness of the side wall insulating film, and a bidirectional Zener diode, formed on the bottom wall insulating film inside the diode trench and having a pair of first conductivity type portions and at least one second conductivity type portion formed between the pair of first conductivity type portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application corresponds to Japanese Patent Application No.2016-42748 filed in the Japan Patent Office on Mar. 4, 2016, JapanesePatent Application No. 2016-242478 filed in the Japan Patent Office onDec. 14, 2016, and Japanese Patent Application No. 2017-22146 filed inthe Japan Patent Office on Feb. 9, 2017 and the entire disclosures ofthese applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device having astructure that includes a semiconductor layer in which a trench isformed and has a bidirectional Zener diode formed inside the trench.

BACKGROUND ART

A semiconductor device that includes a bidirectional Zener diode isdisclosed in Patent Document 1 (Japanese Patent Application PublicationNo. 2001-257349). The semiconductor device includes an n⁺ typesemiconductor substrate. An n⁻ type epitaxial layer is formed on thesemiconductor substrate. A trench is formed in a surface layer portionof the epitaxial layer. A gate oxide film is formed on an inner wall ofthe trench. A bidirectional Zener diode, including an n⁺ type region, ap type region, and an n⁺ type region, is formed in the trench.

SUMMARY OF INVENTION

The semiconductor device according to Patent Document 1 has a structurewhere the p type region of the bidirectional Zener diode faces the n⁻type epitaxial layer across the gate oxide film. Therefore, when avoltage drop occurs across the pair of n⁺ type regions that sandwich thep type region, electrons are drawn to a region, in the p type region,that faces the n⁻ type epitaxial layer. In this case, there is a problemthat an inversion layer, in which the p type is inverted to the n type,is formed in the p type, consequently causing an undesired increase ofcurrent.

An object of the present invention is thus to provide a semiconductordevice with which undesired increase of current can be suppressed in astructure with a bidirectional Zener diode formed inside a trench.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a semiconductor device according to a firstpreferred embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II in FIG. 1.

FIG. 3 is a diagram of a principal portion of a trench gate structure.

FIG. 4 is an enlarged view of a region surrounded by alternate long andtwo short dashes line IV in FIG. 1.

FIG. 5 is a sectional view taken along line V-V in FIG. 4.

FIG. 6 is a diagram of a principal portion of a trench diode structure.

FIG. 7 is a diagram of a principal portion of an electric fieldrelaxation structure.

FIG. 8A to FIG. 8M are sectional views for describing a method formanufacturing the semiconductor device of FIG. 1.

FIG. 9 is a diagram for describing an operation of a bidirectional Zenerdiode.

FIG. 10 is a diagram for describing an operation of a bidirectionalZener diode.

FIG. 11 is a sectional view of a portion corresponding to FIG. 5 andshows a semiconductor device according to a second preferred embodimentof the present invention.

FIG. 12 is a sectional view of a portion corresponding to FIG. 5 andshows a semiconductor device according to a third preferred embodimentof the present invention.

FIG. 13 is a sectional view of a portion corresponding to FIG. 4 andshows a modification example of a bidirectional Zener diode.

FIG. 14 is a plan view of a modification example of a front surfaceelectrode.

FIG. 15 is a schematic plan view of a semiconductor device according toa preferred embodiment of a reference invention.

FIG. 16 is a sectional view taken along section line XVI-XVI in FIG. 15.

FIG. 17 is a diagram of a principal portion of a gate insulating film.

FIG. 18 is an enlarged view of a region surrounded by broken line XVIIIin FIG. 15.

FIG. 19 is a sectional view taken along section line XIX-XIX in FIG. 18.

FIG. 20 is a flow diagram of a method for manufacturing thesemiconductor device.

FIG. 21A to FIG. 21D are sectional views for describing processesrelated to the forming of the gate insulating film.

FIG. 22 is a diagram of gate-source breakdown waveforms.

FIG. 23 is a diagram for describing a mechanism of breakdown of abidirectional Zener diode according to a first mode.

FIG. 24 is a diagram for describing a mechanism of breakdown of abidirectional Zener diode according to a second mode.

FIG. 25 is a diagram for comparing breakdown waveforms of an avalanchedesign and a punch-through design.

FIG. 26 is a diagram for describing how a gate-source breakdown voltageBVgss and electrostatic breakdown resistance vary according to a designdimension of a p⁻ type layer of a bidirectional Zener diode.

FIG. 27 is a diagram showing relationships of area of active region andelectrostatic breakdown resistance according to transistors that differfrom each other in structure.

DESCRIPTION OF EMBODIMENTS

A semiconductor device according to a preferred embodiment of thepresent invention includes a semiconductor layer of a first conductivitytype, having a main surface with a diode trench formed therein, an innerwall insulating film, including a side wall insulating film, formedalong side walls of the diode trench, and a bottom wall insulating film,formed along a bottom wall of the diode trench and having a thicknessgreater than a thickness of the side wall insulating film, and abidirectional Zener diode, formed on the bottom wall insulating filminside the diode trench and having a pair of first conductivity typeportions and at least one second conductivity type portion formedbetween the pair of first conductivity type portions.

A semiconductor device according to another preferred embodiment of thepresent invention includes a semiconductor layer of a first conductivitytype, having a main surface with a diode trench formed therein, an innerwall insulating film, formed along an inner wall of the diode trench, abidirectional Zener diode, formed inside the diode trench and having apair of first conductivity type portions and at least one secondconductivity type portion formed between the pair of first conductivitytype portions, and a floating region of the second conductivity typeformed in a region of the semiconductor layer oriented along a bottomwall of the diode trench.

Effects of Invention

With the semiconductor device according to one preferred embodiment ofthe present invention, the bottom wall insulating film is interposedbetween the bottom wall of the diode trench and the bidirectional Zenerdiode. An inversion suppressing structure that suppresses inversion ofthe conductivity type of the second conductivity type portion of thebidirectional Zener diode to the first conductivity type is formed bythe bottom wall insulating film.

The inversion of the conductivity type of the second conductivity typeportion to the first conductivity type can thereby be suppressed evenwhen a voltage drop occurs across the pair of first conductivity typeportions. A semiconductor device with which undesired increase ofcurrent can be suppressed can thus be provided.

With the semiconductor device according to the other preferredembodiment of the present invention, the inner wall insulating film isinterposed between the bottom wall of the diode trench and thebidirectional Zener diode. In addition, the floating region of thesecond conductivity type, which faces the bidirectional Zener diodeacross the inner wall insulating film, is formed in the semiconductorlayer. An inversion suppressing structure that suppresses the inversionof the conductivity type of the second conductivity type portion of thebidirectional Zener diode to the first conductivity type is formed bythe inner wall insulating film and the floating region.

The inversion of the conductivity type of the second conductivity typeportion to the first conductivity type can thereby be suppressed evenwhen a voltage drop occurs across the pair of first conductivity typeportions. A semiconductor device with which undesired increase ofcurrent can be suppressed can thus be provided.

Preferred embodiments of the present invention shall now be described indetail with reference to the attached drawings.

First Preferred Embodiment

FIG. 1 is a plan view of a semiconductor device 1 according to a firstpreferred embodiment of the present invention.

The semiconductor device 1 according to the present preferred embodimentis a composite type semiconductor device that integrally includes aninsulated gate type field effect transistor Tr and a bidirectional Zenerdiode D. The bidirectional Zener diode D is formed, for example, as aprotecting element that protects the insulated gate type field effecttransistor Tr from overvoltage and overcurrent.

Referring to FIG. 1, the semiconductor device 1 includes a semiconductorlayer 2. The semiconductor layer 2 includes a first main surface 3, asecond main surface 4, positioned at an opposite side of the first mainsurface 3, and side surfaces 5 connecting the first main surface 3 andthe second main surface 4. The semiconductor layer 2 is formed to aquadrilateral chip shape in a plan view of viewing from a direction of anormal to the first main surface 3 (hereinafter referred to simply as“plan view”).

An element forming region 6 and an outer region 7, which is a region atan outer side of the element forming region 6, are set in thesemiconductor layer 2. The element forming region 6 is a region in whichthe insulated gate type field effect transistor Tr and the bidirectionalZener diode D are formed.

In the present preferred embodiment, the element forming region 6 is setto a quadrilateral shape in plan view that has four sides parallel torespective sides of the semiconductor layer 2. The element formingregion 6 is set at an inner side of the semiconductor layer 2 across aninterval from a peripheral edge of the semiconductor layer 2. In thepresent preferred embodiment, the outer region 7 is set to an endlessshape (a quadrilateral annular shape in plan view) in a region betweenside walls of the semiconductor layer 2 and a peripheral edge of theelement forming region 6 so as to surround the element forming region 6.

A front surface electrode 8 is formed on the first main surface 3 of thesemiconductor layer 2. The front surface electrode 8 may, for example,contain at least one type of material among copper, an alloy containingcopper, aluminum, and an alloy containing aluminum. The front surfaceelectrode 8 may, for example, contain an aluminum-copper alloy (Al—Cualloy) or may contain an aluminum-silicon-copper alloy (Al—Si—Cu alloy).

The front surface electrode 8 includes a gate pad 9, a gate finger 10,and a source pad 11. The gate pad 9 and the gate finger 10 form a gateelectrode of the insulated gate type field effect transistor Tr. Thesource pad 11 forms a source electrode of the insulated gate type fieldeffect transistor Tr.

In plan view, the gate pad 9 is formed along one corner portion thatconnects two side surfaces 5 of the semiconductor layer 2. The gate pad9 is formed to a quadrilateral shape in plan view. The gate finger 10 isformed integral to the gate pad 9. In the outer region 7, the gatefinger 10 is formed along a periphery of the element forming region 6.The gate finger 10 is formed to an endless shape (a quadrilateralannular shape in plan view) that surrounds the element forming region 6.

An insulating region 12 is formed inside a region surrounded by the gatepad 9 and the gate finger 10. The insulating region 12 extends along aninner edge of the gate pad 9 and an inner edge of the gate finger 10 andelectrically separates the gate pad 9 and the source pad 11. The sourcepad 11 is formed to an L shape in plan view inside a region surroundedby the insulating region 12.

FIG. 2 is a sectional view taken along line II-II in FIG. 1.

With reference to FIG. 2, the semiconductor layer 2 includes an n⁺ typesemiconductor substrate 21, made of silicon, and an n⁻ type epitaxiallayer 22 formed on a main surface of the n⁺ type semiconductor substrate21. The first main surface 3 of the semiconductor layer 2 is formed bythe n⁻ type epitaxial layer 22, and the second main surface 4 of thesemiconductor layer 2 is formed by the n⁺ type semiconductor substrate21.

An n type impurity concentration of the n⁺ type semiconductor substrate21 is, for example, not less than 1.0×10¹⁹ cm⁻³ and not more than1.0×10²⁰ cm⁻³. An n type impurity concentration of the n⁻ type epitaxiallayer 22 is, for example, not less than 1.0×10¹⁵ cm⁻³ and not more than1.0×10¹⁷ cm⁻³.

A drain electrode 23 is connected to the second main surface 4 of thesemiconductor layer 2. The n⁺ type semiconductor substrate 21 is therebyformed as an n⁺ type drain region 24 and the n⁻ type epitaxial layer 22is formed as an n⁻ type drift drain region 25. The same electrodematerial as that of the front surface electrode 8 may be adopted as theelectrode material of the drain electrode 23.

Trench gate structures 27 are formed in the element forming region 6directly under the source pad 11. The trench gate structures 27 defineunit cells 26 of the insulated gate type field effect transistor Tr.Each trench gate structure 27 includes a gate trench 28, a first innerwall insulating film 29, and an embedded gate electrode 30. In thetrench gate structure 27, the first inner wall insulating film 29 isformed as a gate insulating film.

The gate trenches 28 are formed in the first main surface 3 of thesemiconductor layer 2. The gate trenches 28 may be formed in stripes inplan view or may be formed in a lattice in plan view. A cell pitch P ofthe unit cells 26 that is defined by the distance between mutuallyadjacent gate trenches 28 is, for example, not less than 1.0 μm and notmore than 2.0 μm. Each gate trench 28 has an inner wall that includesside walls and a bottom wall.

The first inner wall insulating film 29 is formed along the inner wallof each gate trench 28. The embedded gate electrode 30 is embedded inthe gate trench 28 across the first inner wall insulating film 29. Thefirst inner wall insulating film 29 may contain silicon oxide (SiO₂).The embedded gate electrode 30 may include polysilicon havingconductivity.

Although unillustrated, the trench gate structures 27 are electricallyconnected to the gate finger 10 in the outer region 7. Power is suppliedto the unit cells 26 by the trench gate structures 27.

FIG. 3 is a diagram of a principal portion of a trench gate structure27.

With reference to FIG. 3, each first inner wall insulating film 29integrally includes a first side wall insulating film 31, a first bottomwall insulating film 32, and a first connection insulating film 33. Thefirst side wall insulating film 31 is formed along the side walls of thecorresponding gate trench 28. The first bottom wall insulating film 32is formed along the bottom wall of the gate trench 28. The firstconnection insulating film 33 is formed along connection portionsconnecting the side walls and the bottom wall of the gate trench 28.

A thickness t1 of the first bottom wall insulating film 32 is greaterthan a thickness t2 of the first side wall insulating film 31 (thicknesst2<thickness t1). A thickness t3 of the first connection insulating film33 is not more than the thickness t2 of the first side wall insulatingfilm 31 (thickness t3≤thickness t2<thickness t1).

A ratio t1/Dg of the thickness t1 of the first bottom wall insulatingfilm 32 with respect to a depth Dg of the gate trench 28 is, forexample, not less than 0.08 and not more than 0.35. A ratio t2/t1 of thethickness t2 of the first side wall insulating film 31 with respect tothe thickness t1 of the first bottom wall insulating film 32 is, forexample, not less than 0.16 and not more than 0.6.

The depth Dg of the gate trench 28 is, for example, not less than 9000 Åand not more than 12000 Å (approximately 10000 Å in the presentpreferred embodiment). The thickness t1 of the first bottom wallinsulating film 32 is, for example, not less than 1000 Å and not morethan 3000 Å. The thickness t2 of the first side wall insulating film 31is, for example, not less than 500 Å and not more than 600 Å. Thethickness t3 of the first connection insulating film 33 is, for example,not less than 400 Å and not more than 600 Å.

Referring to FIG. 2 again, at sides of the trench gate structures 27, ptype body regions 34 are formed in surface layer portions of the firstmain surface 3 of the semiconductor layer 2. Each p type body region 34is shared by mutually adjacent trench gate structures 27. In the presentpreferred embodiment, p type body regions 34 are formed acrosssubstantially the entirety of the first main surface 3 of thesemiconductor layer 2. An impurity concentration of the p type bodyregions 34 is, for example, not less than 1.0×10¹⁵ cm⁻³ and not morethan 1.0×10¹⁷ cm⁻³.

At sides of the trench gate structures 27, n⁺ type source regions 35 areformed in surface layer portions of the p type body regions 34. The n⁺type source regions 35 are exposed from the first main surface 3 of thesemiconductor layer 2. An n type impurity concentration of the n⁺ typesource regions 35 is higher than the n type impurity concentration ofthe n⁻ type epitaxial layer 22 and is, for example, not less than1.0×10¹⁹ cm⁻³ and not more than 1.0×10²⁰ cm⁻³.

At sides of the trench gate structures 27, p⁺ type body contact regions36 are formed in surface layer portions of the p type body regions 34.The p⁺ type body contact regions 36 penetrate through the n⁺ type sourceregions 35 from the first main surface 3 of the semiconductor layer 2and are connected to the p type body regions 34. A p type impurityconcentration of the p⁺ type body contact regions 36 is higher than thep type impurity concentration of the p type body regions 34 and is, forexample, not less than 1.0×10¹⁶ cm⁻³ and not more than 1.0×10¹⁷ cm⁻³.

At sides of the trench gate structures 27, the n⁺ type source regions35, the p type body regions 34, and the n⁻ type epitaxial layer 22 (n⁻type drift drain region 25) are formed successively from the first mainsurface 3 toward the second main surface 4 of the semiconductor layer 2.

The embedded gate electrodes 30 face the n⁺ type source regions 35, thep type body regions 34, and the n⁻ type epitaxial layer 22 across thefirst side wall insulating films 31. In each p type body region 34,regions between the corresponding n⁺ type source regions 35 and the n⁻type epitaxial layer 22 are channels of the insulated gate type fieldeffect transistor Tr.

An insulating layer 40, covering the trench gate structures 27 is formedon the first main surface 3 of the semiconductor layer 2. The insulatinglayer 40 may have a laminated structure formed by laminating a pluralityof insulating films or may have a single layer structure that includesonly a single insulating film. The insulating layer 40 may contain, forexample, silicon oxide (SiO₂) or silicon nitride (SiN).

Source contact holes 41 are formed in the insulating layer 40. Thesource contact holes 41 expose the n⁺ type source regions 35 and the ptype body regions 34 from the insulating layer 40.

The source pad 11 described above is formed over the insulating layer40. The source pad 11 enters into the source contact holes 41 from abovethe insulating layer 40. The source pad 11 is connected to the n⁺ typesource regions 35 and the p⁺ type body contact regions 36 inside thesource contact holes 41.

FIG. 4 is an enlarged view of a region surrounded by alternate long andtwo short dashes line IV in FIG. 1. FIG. 5 is a sectional view takenalong line V-V in FIG. 4. For convenience of description, the gate pad 9and the source pad 11 are indicated by broken lines and a layout of thefirst main surface 3 of the semiconductor layer 2 is indicated by solidlines in FIG. 4.

With reference to FIG. 4 and FIG. 5, a trench diode structure 45 isformed in the element forming region 6 directly under the gate pad 9.The trench diode structure 45 includes a diode trench 46, a second innerwall insulating film 47, and a bidirectional Zener diode D.

The diode trench 46 is formed in a main surface of the semiconductorlayer 2. The diode trench 46 is led out from a region directly under thegate pad 9 to a region directly under the source pad 11. In the presentpreferred embodiment, the diode trench 46 is formed to a rectangularshape in plan view.

The diode trench 46 has a depth Dd substantially equal to the depth Dgof the gate trenches 28. Therefore the depth Dd of the diode trench 46is, for example, not less than 9000 Å and not more than 12000 Å(approximately 10000 Å in the present preferred embodiment).

The diode trench 46 has one end portion positioned in the regiondirectly under the gate pad 9 and another end portion positioned in theregion directly under the source pad 11. The diode trench 46 has aninner wall that includes side walls and a bottom wall.

The second inner wall insulating film 47 is formed along the inner wallof the diode trench 46. The bidirectional Zener diode D is embedded inthe diode trench 46 across the second inner wall insulating film 47.

FIG. 6 is a diagram of a principal portion of the trench diode structure45.

Referring to FIG. 6, the second inner wall insulating film 47 has astructure substantially similar to that of each first inner wallinsulating film 29. More specifically, the second inner wall insulatingfilm 47 integrally includes a second side wall insulating film 48, asecond bottom wall insulating film 49, and a second connectioninsulating film 50. The second side wall insulating film 48 is formedalong the side walls of the diode trench 46. The second bottom wallinsulating film 49 is formed along the bottom wall of the diode trench46. The second connection insulating film 50 is formed along connectionportions connecting the side walls and the bottom wall of the diodetrench 46.

A thickness t4 of the second bottom wall insulating film 49 is greaterthan a thickness t5 of the second side wall insulating film 48(thickness t5<thickness t4). A thickness t6 of the second connectioninsulating film 50 is not more than the thickness t5 of the second sidewall insulating film 48 (thickness t6≤thickness t5<thickness t4).

A ratio t4/Dd of the thickness t4 of the second bottom wall insulatingfilm 49 with respect to the depth Dd of the diode trench 46 is, forexample, not less than 0.08 and not more than 0.35. A ratio t5/t4 of thethickness t5 of the second side wall insulating film 48 with respect tothe thickness t4 of the second bottom wall insulating film 49 is, forexample, not less than 0.16 and not more than 0.6.

In the present preferred embodiment, the thickness t4 of the secondbottom wall insulating film 49 is substantially equal to the thicknesst1 of the first bottom wall insulating films 32 (thickness t4=thicknesst1 or thickness t4

thickness t1). In the present preferred embodiment, the thickness t5 ofthe second side wall insulating film 48 is substantially equal to thethickness t2 of the first side wall insulating films 31 (thicknesst5=thickness t2 or thickness t5

thickness t2). In the present preferred embodiment, the thickness t6 ofthe second connection insulating film 50 is substantially equal to thethickness t3 of the first connection insulating films 33 (thicknesst6=thickness t3 or thickness t6

thickness t3).

Referring again to FIG. 4 and FIG. 5, the bidirectional Zener diode D isformed on the second bottom wall insulating film 49 inside the diodetrench 46. In the present preferred embodiment, the bidirectional Zenerdiode D is formed to a rectangular shape extending along the diodetrench 46 in plan view. The bidirectional Zener diode D has one endportion positioned in the region directly under the gate pad 9 andanother end portion positioned in the region directly under the sourcepad 11.

The bidirectional Zener diode D has a flat upper surface 51 facing anopening of the diode trench 46. The upper surface 51 of thebidirectional Zener diode D is formed substantially parallel to thebottom wall of the diode trench 46.

In the direction of the normal to the first main surface 3 of thesemiconductor layer 2, a distance between the first main surface 3 ofthe semiconductor layer 2 and the bottom wall of the diode trench 46 issubstantially equal to a distance between the upper surface 51 of thebidirectional Zener diode D and the bottom wall of the diode trench 46.The upper surface 51 of the bidirectional Zener diode D is thus formedon the same plane as the first main surface 3 of the semiconductor layer2.

The bidirectional Zener diode D is formed across an interval from theside walls of the diode trench 46. Side walls of the bidirectional Zenerdiode D are formed inside a region surrounded by the side walls of thediode trench 46. A distance between the side walls of the bidirectionalZener diode D and the side walls of the diode trench 46 is greater thana thickness of the bidirectional Zener diode D.

The bidirectional Zener diode D includes n⁺ type portions 52 (firstconductivity type portions) and p type portions 53 (second conductivitytype portions) and has a structure in which the n⁺ type portions 52 andthe p type portion 53 are repeated alternately. The n⁺ type portions 52are respectively formed at the one end portion and the other end portionof the bidirectional Zener diode D. The n⁺ type portions 52 and the ptype portions 53 are formed alternately repeatedly in a region betweenthe pair of n⁺ type portions 52 formed at the respective end portions ofthe bidirectional Zener diode D.

In the present preferred embodiment, the n⁺ type portions 52 and the ptype portions 53 are formed in bands, which, in plan view, extend alongan intersecting direction intersecting a direction in which the diodetrench 46 extends. The n⁺ type portions 52 and the p type portions 53are thereby formed in stripes extending along the intersectingdirection. The intersecting direction may be an orthogonal directionorthogonal to the direction in which the diode trench 46 extends.

A pn junction portion is formed in each region between an n⁺ typeportion 52 and a p type portion 53. Zener diodes DZ1 and DZ2, eachhaving an n⁺ type portion 52 as a cathode and a p type portion 53 as ananode, are formed by the pn junction portions.

The bidirectional Zener diode D includes a plurality (four in thepresent preferred embodiment) of bidirectional Zener diode elements DE.Each bidirectional Zener diode element DE includes a pair of the Zenerdiodes DZ1 and DZ2 that are electrically connected to each other via ananode (a p type portion 53).

Mutually adjacent bidirectional Zener diode elements DE are electricallyconnected to each other via a cathode (an n⁺ type portion 52). In thepresent preferred embodiment, a single bidirectional Zener diode D isformed by such a plurality of bidirectional Zener diode elements DE.

The bidirectional Zener diode D may have structure that includes onlyone bidirectional Zener diode element DE. The bidirectional Zener diodeD may thus have a pair of n⁺ type portions 52 and at least one p typeportion 53 formed between the pair of n⁺ type portions 52.

In the present preferred embodiment, the bidirectional Zener diode Dincludes a polysilicon body 54. In the present preferred embodiment,each n⁺ type portion 52 includes an n⁺ type impurity region formed byselectively implanting an n type impurity into the polysilicon body 54.In the present preferred embodiment, each p type portion 53 includes a ptype impurity region formed by selectively implanting a p type impurityinto the polysilicon body 54.

The n⁺ type portions 52 may have an n type impurity concentrationsubstantially equal to the n type impurity concentration of the n⁺ typesource regions 35. The p type portions 53 may have a p type impurityconcentration substantially equal to the p type impurity concentrationof the p type body regions 34.

Referring to FIG. 5, a p type floating region 55 is formed in a regionof the semiconductor layer 2 along the bottom wall of the diode trench46. The p type floating region 55 is formed along portions of the sidewalls of the diode trench 46 in addition to the bottom wall of the diodetrench 46. The p type floating region 55 also covers corner portionsconnecting the side walls and the bottom wall of the diode trench 46. Ap type impurity concentration of the p type floating region 55 is, forexample, not less than 1.0×10¹⁵ cm⁻³ and not more than 1.0×10¹⁷ cm⁻³.

In the present preferred embodiment, the second bottom wall insulatingfilm 49 is interposed between the bottom wall of the diode trench 46 andthe bidirectional Zener diode D. In addition, the p type floating region55, facing the bidirectional Zener diode D across the second bottom wallinsulating film 49, is formed in the semiconductor layer 2. An inversionsuppressing structure 56 that suppresses inversion of the conductivitytype of the p type portions 53 of the bidirectional Zener diode D to then type is formed by the second bottom wall insulating film 49 and the ptype floating region 55.

Referring to FIG. 5, a side wall protection film 57, with an insulatingproperty, is formed on the side walls of the bidirectional Zener diodeD. The side wall protection film 57 fills a region between the sidewalls of the bidirectional Zener diode D and the side walls of the diodetrench 46. The bidirectional Zener diode D is protected from the sidewall sides by the side wall protection film 57. Also, by the side wallprotection film 57, an insulating property between the bidirectionalZener diode D and the semiconductor layer 2 is improved in regard to alateral direction parallel to the first main surface 3 of thesemiconductor layer 2.

Referring to FIG. 4 and FIG. 5, in the element forming region 6,electric field relaxation structures 61, each of which relaxes anelectric field in a peripheral region along a peripheral edge of thediode trench 46, are formed in the peripheral region. In the presentpreferred embodiment, the electric field relaxation structures 61include a plurality (four, in the present preferred embodiment) ofelectric field relaxation structures 61A, 61B, 61C, and 61D formed atintervals in that order in a direction away from the diode trench 46.

The electric field relaxation structures 61A, 61B, 61C, and 61D areformed so as to surround the diode trench 46. Each of the electric fieldrelaxation structures 61A, 61B, 61C, and 61D includes an electric fieldrelaxation trench 62, a third inner wall insulating film 63, an embeddedconductor 64, and a p type floating region 65.

Each electric field relaxation trench 62 is formed in the first mainsurface 3 of the semiconductor layer 2. In the present preferredembodiment, the electric field relaxation trench 62 is formed to anendless shape in plan view (a quadrilateral annular shape in plan view)that surrounds a periphery of the diode trench 46.

The electric field relaxation trench 62 has a depth De substantiallyequal to the depth Dg of the gate trenches 28 and the depth Dd of thediode trench 46. Therefore the depth De of the electric field relaxationtrench 62 is, for example, not less than 9000 Å and not more than 12000Å (approximately 10000 Å in the present preferred embodiment). Theelectric field relaxation trench 62 has an inner wall that includes sidewalls and a bottom wall.

The third inner wall insulating film 63 is formed along the inner wallof the electric field relaxation trench 62. The embedded conductor 64 isembedded in the electric field relaxation trench 62 across the thirdinner wall insulating film 63. The third inner wall insulating film 63may contain silicon oxide. The embedded conductor 64 may containpolysilicon having conductivity.

FIG. 7 is a diagram of a principal portion of an electric fieldrelaxation structure 61.

Each third inner wall insulating film 63 has a structure substantiallysimilar to those of each first inner wall insulating film 29 and thesecond inner wall insulating film 47. More specifically, the third innerwall insulating film 63 integrally includes a third side wall insulatingfilm 66, a third bottom wall insulating film 67, and a third connectioninsulating film 68.

The third side wall insulating film 66 is formed along the side walls ofthe electric field relaxation trench 62. The third bottom wallinsulating film 67 is formed along the bottom wall of the electric fieldrelaxation trench 62. The third connection insulating film 68 is formedalong connection portions connecting the side walls and the bottom wallof the electric field relaxation trench 62.

A thickness t7 of the third bottom wall insulating film 67 is greaterthan a thickness t8 of the third side wall insulating film 66 (thicknesst8<thickness t7). A thickness t9 of the third connection insulating film68 is not more than the thickness t8 of the third side wall insulatingfilm 66 (thickness t9≤thickness t8<thickness t7).

A ratio t7/De of the thickness t7 of the third bottom wall insulatingfilm 67 with respect to the depth De of the electric field relaxationtrench 62 is, for example, not less than 0.08 and not more than 0.35. Aratio t8/t7 of the thickness t8 of the third side wall insulating film66 with respect to the thickness t7 of the third bottom wall insulatingfilm 67 is, for example, not less than 0.16 and not more than 0.6.

In the present preferred embodiment, the thickness t7 of the thirdbottom wall insulating film 67 is substantially equal to the thicknesst4 of the second bottom wall insulating film 49 (thickness t7=thicknesst4 or thickness t7

thickness t4). In the present preferred embodiment, the thickness t8 ofthe third side wall insulating film 66 is substantially equal to thethickness t5 of the second side wall insulating film 48 (thicknesst8=thickness t5 or thickness t8

thickness t5). In the present preferred embodiment, the thickness t9 ofthe third connection insulating film 68 is substantially equal to thethickness t6 of the second connection insulating film 50 (thicknesst9=thickness t6 or thickness t9

thickness t6).

Referring again to FIG. 5, each p type floating region 65 is formed in aregion of the semiconductor layer 2 along the bottom wall of theelectric field relaxation trench 62. The p type floating region 65 isformed along portions of the side walls of the electric field relaxationtrench 62 in addition to the bottom wall of the electric fieldrelaxation trench 62. The p type floating region 65 thus also coverscorner portions connecting the side walls and the bottom wall of theelectric field relaxation trench 62.

The p type floating regions 65 may have a p type impurity concentrationsubstantially equal to the p type impurity concentration of the p typefloating region 55 at the diode trench 46 side. Also, the p typefloating regions 65 may be formed to a depth substantially equal to thedepth of the p type floating region 55. Electric fields at bottomportions of the electric field relaxation trenches 62, especially, theelectric fields at corner portions of the electric field relaxationtrenches 62 can be relaxed by the p type floating regions 65.

The number and shapes of the electric field relaxation structures 61 maybe changed as suited in accordance with the electric field to berelaxed. Therefore, a structure where only one electric field relaxationstructure 61 is formed may be adopted or a structure where eight or moreelectric field relaxation structures 61 are formed may be adopted. Also,dot-shaped or line-shaped electric field relaxation structures 61 thatare discontinuous may be formed so as to surround the peripheral edge ofthe diode trench 46.

The trench diode structure 45 and the electric field relaxationstructures 61 are covered by the insulating layer 40 described above.The side wall protection film 57 formed in the trench diode structure 45may be formed by a portion of the insulating layer 40. A first contacthole 71 and a second contact hole 72 are formed in the insulating layer40.

The first contact hole 71 exposes the one end portion (n⁺ type portion52) of the bidirectional Zener diode D that is positioned directly belowthe gate pad 9. A bottom portion of the first contact hole 71 may bepositioned inside the one end portion of the bidirectional Zener diodeD.

The second contact hole 72 exposes the other end portion (n⁺ typeportion 52) of the bidirectional Zener diode D that is positioneddirectly below the source pad 11. A bottom portion of the second contacthole 72 may be positioned inside the other end portion of thebidirectional Zener diode D.

A first contact plug 73 is embedded in the first contact hole 71. Thefirst contact plug 73 is electrically connected to the gate pad 9 andthe one end portion (n⁺ type portion 52) of the bidirectional Zenerdiode D. The first contact plug 73 may contain tungsten (W).

A second contact plug 74 is embedded in the second contact hole 72. Thesecond contact plug 74 is electrically connected to the source pad 11and the other end portion (n⁺ type portion 52) of the bidirectionalZener diode D. The second contact plug 74 may contain tungsten (W).

An example of a method for manufacturing the semiconductor device 1shall now be described. FIG. 8A to FIG. 8M are sectional views fordescribing the method for manufacturing the semiconductor device 1 ofFIG. 1. FIG. 8A to FIG. 8M are sectional views of the portioncorresponding to FIG. 5 described above. With FIG. 8A to FIG. 8M, adescription mainly focusing on the trench diode structure 45 and thestructure of the periphery thereof shall be provided.

First, with reference to FIG. 8A, the n⁺ type semiconductor substrate 21is prepared. Next, silicon is grown epitaxially from a main surface ofthe semiconductor substrate 21 while introducing an n type impurity.Then n⁻ type epitaxial layer 22 is thereby formed on the main surface ofthe n⁺ type semiconductor substrate 21. The semiconductor layer 2 isformed by the laminated structure of the n⁺ type semiconductor substrate21 and the n⁻ type epitaxial layer 22. The semiconductor layer 2 has thefirst main surface 3 and the second main surface 4.

Next, with reference to FIG. 8B, a mask 81 is formed on the first mainsurface 3 of the semiconductor layer 2. The mask 81 may be a siliconoxide film formed by selectively oxidizing the first main surface 3 ofthe semiconductor layer 2. The mask 81 selectively has openings 82exposing regions in which the gate trenches 28, the diode trench 46, andthe electric field relaxation trenches 62 are to be formed.

Next, unnecessary portions of the semiconductor layer 2 are removedselectively by etching via the mask 81. The gate trenches 28, the diodetrench 46, and the electric field relaxation trenches 62 are therebyformed in the first main surface 3 of the semiconductor layer 2.

Next, with reference to FIG. 8C, a p type impurity is introduced intoportions of the semiconductor layer 2 exposed at bottom portions of thediode trench 46 and the electric field relaxation trenches 62. The ptype impurity is, for example, implanted into the semiconductor layer 2via an unillustrated ion implantation mask. The p type floating region55 is thereby formed along the bottom portion of the diode trench 46,and the p type floating regions 65 are formed along the bottom portionsof the electric field relaxation trenches 62.

Next, with reference to FIG. 8D, an insulating material layer 83 isformed, for example, by a CVD (chemical vapor deposition) method. Theinsulating material layer 83 is formed so as to fill the gate trenches28, the diode trench 46, and the electric field relaxation trenches 62and cover substantially the entirety of the first main surface 3 of thesemiconductor layer 2.

Next, with reference to FIG. 8E, unnecessary portions of the insulatingmaterial layer 83 are removed by etching back to intermediate portionsin a depth direction of the gate trenches 28, the diode trench 46, andthe electric field relaxation trenches 62. The first bottom wallinsulating films 32, the second bottom wall insulating film 49, and thethird bottom wall insulating films 67 are thereby formed.

Next, with reference to FIG. 8F, portions of the semiconductor layer 2exposed at the side walls of the gate trenches 28, the side walls of thediode trench 46, and the side walls of the electric field relaxationtrenches 62 are oxidized, for example, by a thermal oxidation method ora wet oxidation method. The first side wall insulating films 31, thesecond side wall insulating film 48, and the third side wall insulatingfilms 66 are thereby formed. The first connection insulating films 33,the second connection insulating film 50, and the third connectioninsulating films 68 are also formed thereby.

Next, with reference to FIG. 8G, a polysilicon layer 84 is formed, forexample, by the CVD method. The polysilicon layer 84 is formed so as tofill the gate trenches 28, the diode trench 46, and the electric fieldrelaxation trenches 62 and cover substantially the entirety of the firstmain surface 3 of the semiconductor layer 2.

Next, a mask 86, selectively covering a flat region 85, positionedinside the diode trench 46, is formed in the polysilicon layer 84. Astep portion 87 is formed between a portion of the polysilicon layer 84formed inside the diode trench 46 and a portion of the polysilicon layer84 formed on the first main surface 3 of the semiconductor layer 2. Theflat region 85 refers to a flat region surrounded by the step portion 87in the portion of the polysilicon layer 84 positioned inside the diodetrench 46.

Next, with reference to FIG. 8H, unnecessary portions of the polysiliconlayer 84 are removed by etching back via the mask 86. The embedded gateelectrodes 30 are thereby formed inside the first bottom wall insulatingfilms 32. Also, the polysilicon body 54, which is to be a basis of thebidirectional Zener diode D, is formed inside the diode trench 46. Also,the embedded conductors 64 are formed inside the electric fieldrelaxation trenches 62.

In this process, the polysilicon body 54, having the flat upper surface51 facing the opening of the diode trench 46, is formed. In thedirection of the normal to the first main surface 3 of the semiconductorlayer 2, the distance between the first main surface 3 of thesemiconductor layer 2 and the bottom wall of the diode trench 46 issubstantially equal to a distance between the upper surface 51 of thepolysilicon body 54 and the bottom wall of the diode trench 46. Theupper surface 51 of the polysilicon body 54 is thus formed onsubstantially the same plane as the first main surface 3 of thesemiconductor layer 2.

Also in this process, the polysilicon body 54 is formed inside the diodetrench 46 across an interval from the side walls of the diode trench 46.Side walls of the polysilicon body 54 are formed inside the regionsurrounded by the side walls of the diode trench 46. A distance betweenthe side walls of the polysilicon body 54 and the side walls of thediode trench 46 is formed to be greater than a thickness of thepolysilicon body 54.

The step portion 87, present between the portion of the polysiliconlayer 84 formed on the first main surface 3 of the semiconductor layer 2and the portion of the polysilicon layer 84 formed inside the diodetrench 46, can thereby be removed. Remaining of the step portion 87 as aportion of the polysilicon body 54 can thus be suppressed and thebidirectional Zener diode D, having the flat upper surface 51, can thusbe formed.

Next, with reference to FIG. 8I, a photomask 88 is formed on the firstmain surface 3 of the semiconductor layer 2. The photomask 88 may be ofa negative type or may be of a positive type. Here, a description shallbe provided with a negative type photomask 88 as an example.

Next, an opening 89, exposing a region in which the p type body regions34 are to be formed, and an opening 90, exposing a region in which the ptype portions 53 of the polysilicon body 54 are to be formed, areselectively formed in the photomask 88 by exposure and development.

Next, the p type impurity is implanted via the photomask 88 intoentireties of the polysilicon body 54 and a surface layer portion of thefirst main surface 3 of the semiconductor layer 2. The p type impurityimplanted into the surface layer portion of the first main surface 3 ofthe semiconductor layer 2 becomes the p type body regions 34. The p typeimpurity implanted into the polysilicon body 54 becomes the p typeportions 53 after a process to be described below. After implantation ofthe p type impurity, the photomask 88 is removed.

Here, a case where a comparatively large step is present in thephotomask 88 between a portion covering the upper surface 51 of thepolysilicon body 54 and a portion covering the first main surface 3 ofthe semiconductor layer 2 shall be considered.

In exposing such a photomask 88, different focus margins must be setrespectively for the portion covering the upper surface 51 of thepolysilicon body 54 and the portion covering the first main surface 3 ofthe semiconductor layer 2. It thus becomes quite unrealistic to performexposure of the portion covering the upper surface 51 of the polysiliconbody 54 and exposure of the portion covering the first main surface 3 ofthe semiconductor layer 2 in the same process.

The focus margin is a width of a depth region in which a photomask canbe maintained in a practically usable state when a focal point of lightwith respect to the photomask deviates upwards or downwards from anoptimal focal point position during exposure.

On the other hand, with the present preferred embodiment, the uppersurface 51 of the polysilicon body 54 and the first main surface 3 ofthe semiconductor layer 2 are formed on substantially the same plane.Forming of a step in the photomask 88 between the portion covering theupper surface 51 of the polysilicon body 54 and the portion covering thefirst main surface 3 of the semiconductor layer 2 can thus besuppressed. Moreover, the upper surface 51 of the polysilicon body 54 isformed to be flat and therefore the forming of a step in the photomask88 above the upper surface 51 of the polysilicon body 54 can also besuppressed.

An equal focus margin can thus be set respectively for the portioncovering the upper surface 51 of the polysilicon body 54 and the portioncovering the first main surface 3 of the semiconductor layer 2 whenexposing the photomask 88.

A process of forming the p type body regions 34 and a process of formingthe p type portions 53 of the polysilicon body 54 can thereby beperformed as a process in common. Also, at the same time, the process offorming the p type portions 53 in the polysilicon body 54 formed insidethe diode trench 46 can be simplified.

Next, with reference to FIG. 8J, another photomask 91 is formed on thefirst main surface 3 of the semiconductor layer 2. The photomask 91 maybe of a negative type or may be of a positive type. Here, a descriptionshall be provided with a negative type photomask 91 as an example.

Next, openings (not shown), exposing regions in which the n⁺ type sourceregions 35 are to be formed, and openings 92, exposing regions in whichthe n⁺ type portion 52 of the polysilicon body 54 are to be formed, areselectively formed in the photomask 91 by exposure and development.

Next, the n type impurity is implanted via the photomask 91 into thepolysilicon body 54 and surface layer portions of the first main surface3 of the semiconductor layer 2. The n type impurity implanted into thesurface layer portions of the first main surface 3 of the semiconductorlayer 2 becomes the n⁺ type source regions 35. The n type impurityimplanted into the polysilicon body 54 becomes the n⁺ type portions 52.

By the present process, the bidirectional Zener diode D that includesthe n⁺ type portions 52 and the p type portions 53 and has the structurein which the n⁺ type portions 52 and the p type portion 53 are repeatedalternately is formed inside the diode trench 46. After implantation ofthe n type impurity, the photomask 91 is removed.

In the present process, forming of a step in the photomask 91 between aportion covering the upper surface 51 of the polysilicon body 54 and aportion covering the first main surface 3 of the semiconductor layer 2can be suppressed. Moreover, the upper surface 51 of the polysiliconbody 54 is formed to be flat and therefore the forming of a step in thephotomask 91 above the upper surface 51 of the polysilicon body 54 canalso be suppressed.

An equal focus margin can thus be set respectively for the portioncovering the upper surface 51 of the polysilicon body 54 and the portioncovering the first main surface 3 of the semiconductor layer 2 whenexposing the photomask 91.

A process of forming the n⁺ type source regions 35 and a process offorming the n⁺ type portions 52 of the polysilicon body 54 can therebybe performed as a process in common. Also, at the same time, the processof forming the n⁺ type portions 52 in the polysilicon body 54 formedinside the diode trench 46 can be simplified.

Next, with reference to FIG. 8K, the insulating layer 40 is formed, forexample, by the CVD method. The insulating layer 40 is formed to fill aregion between the side walls of the polysilicon body 54 and the sidewalls of the diode trench 46 and cover substantially the entirety of thefirst main surface 3 of the semiconductor layer 2. A portion of theinsulating layer 40 that fills the region between the side walls of thepolysilicon body 54 and the side walls of the diode trench 46 becomesthe side wall protection film 57 that protects the side walls of thepolysilicon body 54.

Next, with reference to FIG. 8L, a mask 93 is formed on the insulatinglayer 40. The mask 93 has openings 94 that selectively expose regions inwhich the first contact hole 71 and the second contact hole 72 are to beformed.

Next, unnecessary portions of the insulating layer 40 are removed byetching via the mask 98. The first contact hole 71, exposing one endportion of the polysilicon body 54, and the second contact hole 72,exposing another end portion of the polysilicon body 54, are therebyformed in the insulating layer 40. After the first contact hole 71 andthe second contact hole 72 are formed, the mask 93 is removed.

Next, with reference to FIG. 8M, tungsten is embedded in the firstcontact hole 71 and the second contact hole 72, for example, by the CVDmethod and etching back. The first contact plug 73 is thereby formedinside the first contact hole 71. Also, the second contact plug 74 isthereby formed inside the second contact hole 72.

Next, an electrode material (for example, aluminum) is deposited on theinsulating layer 40, for example, by a sputtering method to form anelectrode material layer. Next, unnecessary portions of the electrodematerial layer are removed, for example, by etching via a mask (notshown). The front surface electrode 8, which includes the gate pad 9,the gate finger 10, and the source pad 11, is thereby formed.Thereafter, the drain electrode 23 is formed on the second main surface4 of the semiconductor layer 2, for example, by the sputtering method.The semiconductor device 1 is obtained through the above processes.

Next, operations of the bidirectional Zener diode D shall be described.Here, two bidirectional Zener diodes D that were adjusted in a width Wpof each p type portion 53 were prepared and the respective operationswere examined.

FIG. 9 and FIG. 10 are respectively diagrams for describing operationsof the bidirectional Zener diodes D. FIG. 9 and FIG. 10 respectivelyillustrate the operations when the corresponding bidirectional Zenerdiodes D are made to undergo breakdown due to avalanche breakdown.

With the bidirectional Zener diode D of FIG. 9, the width Wp of each ptype portion 53 is set to a value greater than a width Wd of a depletionlayer spreading from each pn junction portion formed between an n⁺ typeportion 52 and a p type portion 53 (width Wp>width Wd). The width Wd ofthe depletion layer is the width of the depletion layer that spreadsfrom each pn junction portion when a breakdown voltage is applied acrossthe gate pad 9 and the source pad 11.

Referring to FIG. 9, when the breakdown voltage is applied across thegate pad 9 and the source pad 11, the bidirectional Zener diode D ismade conductive due to avalanche breakdown. A gate current, whichcontains a noise component, thus flows across the gate pad 9 and thesource pad 11 via the bidirectional Zener diode D.

The width Wp of the p type portion 53 is set to the value greater thanthe width Wd of the depletion layer (width Wp>width Wd). Therefore, inthe breakdown state, each p type portion 53 is not filled up by thedepletion layer and a portion of the p type portion 53 remains at afixed width.

The remaining portion of the p type portion 53 becomes a serial resistorand therefore becomes an obstruction to releasing the gate current to aground potential (gate pad 9). Consequently, a problem, such asbreakdown of the gate insulating film, etc., occurs.

On the other hand, with the bidirectional Zener diode D shown in FIG.10, the width Wp of each p type portion 53 is set to a value not greaterthan the width Wd of the depletion layer spreading from each pn junctionportion formed between an n⁺ type portion 52 and a p type portion 53(width Wp≤width Wd).

Therefore, when the breakdown voltage is applied across the gate pad 9and the source pad 11, the region of each p type portion 53 can becomefilled up by the depletion layer. The n⁺ type portion 52 at the sourceside and the n⁺ type portion 52 at the gate side can thereby be madeconductive to each other by punch-through and the serial resistor formedby the p type portions 53 can be reduced.

In the present preferred embodiment, the inversion suppressing structure56, including the second bottom wall insulating film 49 and the floatingregion, is formed directly under the bidirectional Zener diode D (seeFIG. 5). The conductivity type of the p type portions 53 of thebidirectional Zener diode D is suppressed from inverting to the n typeby the inversion suppressing structure 56. Forming of an undesiredcurrent path between the gate pad 9 and the source pad 11 can thus besuppressed. Stability of on/off operations by the bidirectional Zenerdiode D can thus be improved.

The gate current can thereby be released satisfactorily to the groundpotential (gate pad 9). A problem, such as breakdown of the gateinsulating film, etc., can thus be suppressed and electrostaticbreakdown resistance and avalanche resistance can be improved. Theavalanche resistance refers to resistance that keeps the bidirectionalZener diode D from breaking down in an avalanche breakdown state.

As described above, with the semiconductor device 1 according to thepresent preferred embodiment, the second bottom wall insulating film 49is interposed between the bottom wall of the diode trench 46 and thebidirectional Zener diode D. The thickness t4 of the second bottom wallinsulating film 49 is greater than the thickness t5 of the second sidewall insulating film 48.

In addition, the p type floating region 55, which faces thebidirectional Zener diode D across the second bottom wall insulatingfilm 49, is formed in the semiconductor layer 2. The inversionsuppressing structure 56 that suppresses the inversion of theconductivity type of the p type portions 53 of the bidirectional Zenerdiode D to the n type is formed by the second bottom wall insulatingfilm 49 and the floating region.

The inversion of the conductivity type of the p type portions 53 to then type can thereby be suppressed even when a voltage drop occurs acrossa pair of the n⁺ type portions 52. Increase of an undesired current,such as a leakage current, etc., can thus be suppressed in thebidirectional Zener diode D. The stability of on/off operations by thebidirectional Zener diode D can thus be improved and therefore thestability of on/off operations can be made to contribute to improvementof electrostatic breakdown resistance and improvement of avalancheresistance.

Also with the semiconductor device 1 according to the present preferredembodiment, the bidirectional Zener diode D has the upper surface 51facing the opening of the diode trench 46 and the upper surface 51 ofthe bidirectional Zener diode D is formed on the same plane as the firstmain surface 3 of the semiconductor layer 2.

By the above, in the photomask 88 used in the process of forming the ptype body regions 34 and the p type portions 53, a step is suppressedfrom forming between the portion covering the upper surface 51 of thepolysilicon body 54 and the portion covering the first main surface 3 ofthe semiconductor layer 2 (see FIG. 8I).

Moreover, the upper surface 51 of the polysilicon body 54 is formed tobe flat above the bottom wall of the diode trench 46. The forming of astep in the photomask 88 above the upper surface 51 of the polysiliconbody 54 can also be suppressed thereby.

An equal focus margin can thus be set respectively for the portioncovering the upper surface 51 of the polysilicon body 54 and the portioncovering the first main surface 3 of the semiconductor layer 2 whenexposing the photomask 88. The process of forming the p type bodyregions 34 and the process of forming the p type portions 53 of thepolysilicon body 54 can thereby be performed as a process in common.Also, at the same time, the process of forming the p type portions 53 inthe polysilicon body 54 formed inside the diode trench 46 can besimplified.

Also, in the photomask 91 used in the process of forming the n⁺ typesource regions 35 and the n⁺ type portions 52, a step is suppressed fromforming between the portion covering the upper surface 51 of thepolysilicon body 54 and the portion covering the first main surface 3 ofthe semiconductor layer 2 (see FIG. 8J).

Moreover, the upper surface 51 of the polysilicon body 54 is formed tobe flat above the bottom wall of the diode trench 46. The forming of astep in the photomask 91 above the upper surface 51 of the polysiliconbody 54 can also be suppressed thereby.

An equal focus margin can thus be set respectively for the portioncovering the upper surface 51 of the polysilicon body 54 and the portioncovering the first main surface 3 of the semiconductor layer 2 whenexposing the photomask 91. The process of forming the n⁺ type sourceregions 35 and the process of forming the n⁺ type portions 52 of thepolysilicon body 54 can thereby be performed as a process in common.Also, at the same time, the process of forming the n⁺ type portions 52in the polysilicon body 54 formed inside the diode trench 46 can besimplified.

Also with the semiconductor device 1 according to the present preferredembodiment, the bidirectional Zener diode D is formed inside the diodetrench 46 across an interval from the side walls of the diode trench 46.The distance between the side walls of the bidirectional Zener diode Dand the side walls of the diode trench 46 is greater than the thicknessof the bidirectional Zener diode D.

With the present arrangement, the step portion 87, present between theportion of the polysilicon layer 84 formed on the first main surface 3of the semiconductor layer 2 and the portion of the polysilicon layer 84formed inside the diode trench 46 in the processes of FIG. 8G to FIG. 8Hdescribed above, can be removed. The step portion 87 can thereby besuppressed from remaining as a portion of the polysilicon body 54 andthe bidirectional Zener diode D, having the flat upper surface 51, canthus be formed.

Also, the semiconductor device 1 according to the present preferredembodiment includes the side wall protection film 57 with the insulatingproperty that is formed in the region between the side walls of thediode trench 46 and the side walls of the bidirectional Zener diode Dand protects the side walls of the bidirectional Zener diode D. The sidewall protection film 57 fills the region between the side walls of thediode trench 46 and the side walls of the bidirectional Zener diode D.

With the present arrangement, the bidirectional Zener diode D isprotected from the side wall sides by the side wall protection film 57.Also, by the side wall protection film 57, the insulating propertybetween the bidirectional Zener diode D and the semiconductor layer 2can be improved in regard to the lateral direction parallel to the firstmain surface 3 of the semiconductor layer 2. Electric field influencesthat the bidirectional Zener diode D applies to the semiconductor layer2 can thus be reduced.

The semiconductor device 1 according to the present preferred embodimentalso includes the electric field relaxation structures 61, which, in theperipheral region along the peripheral edge of the diode trench 46, areformed in the surface layer portion of the main surface of thesemiconductor layer 2 and relax the electric field in the peripheralregion.

With the present arrangement, concentration of electric field in theperipheral region along the peripheral edge of the diode trench 46 canbe suppressed by the electric field relaxation structures 61. Loweringof the electrostatic breakdown resistance and lowering of the avalancheresistance due to concentration of electric field can thus besuppressed.

Also, with the semiconductor device 1 according to the present preferredembodiment, the trench gate structures 27, the trench diode structure45, and the electric field relaxation structures 61 have substantiallythe same arrangement. The trench gate structures 27, the trench diodestructure 45, and the electric field relaxation structures 61 can thusbe formed inside the semiconductor layer 2 by processes in common.Simplification of the manufacturing process and reduction of workloadcan thus be achieved.

Second Preferred Embodiment

FIG. 11 is a sectional view of a portion corresponding to FIG. 5 andshows a semiconductor device 95 according to a second preferredembodiment of the present invention. In FIG. 11, arrangements that arethe same as the arrangements described above with the first preferredembodiment are provided with the same reference symbols and descriptionthereof shall be omitted.

With the exception of the point of not including the p type floatingregion 55 along the bottom wall of the diode trench 45, thesemiconductor device 95 according to the present preferred embodimenthas the same arrangement as that of the semiconductor device 1 accordingto the first preferred embodiment described above.

With the semiconductor device 95 according to the present preferredembodiment, the inversion suppressing structure 56, which suppresses theinversion of the conductivity type of the p type portions 53 of thebidirectional Zener diode D to the n type, is formed by the secondbottom wall insulating film 49 that is interposed between the bottomwall of the diode trench 46 and the bidirectional Zener diode D.

The same actions and effects as the actions and effects described abovefor the first preferred embodiment can be exhibited by such anarrangement as well.

Third Preferred Embodiment

FIG. 12 is a sectional view of a portion corresponding to FIG. 5 andshows a semiconductor device 97 according to a third preferredembodiment of the present invention. In FIG. 12, arrangements that arethe same as the arrangements described above with the first preferredembodiment are provided with the same reference symbols and descriptionthereof shall be omitted.

With the exception of the point of that the thickness t4 of the secondbottom wall insulating film 49 is formed to be substantially equal tothe thickness t5 of the second side wall insulating film 48, thesemiconductor device 97 according to the present preferred embodimenthas the same arrangement as that of the semiconductor device 1 accordingto the first preferred embodiment described above.

With the semiconductor device 97 according to the present preferredembodiment, the inversion suppressing structure 56, which suppresses theinversion of the conductivity type of the p type portions 53 of thebidirectional Zener diode D to the n type, is formed by the thin secondbottom wall insulating film 49, which is interposed between the bottomwall of the diode trench 46 and the bidirectional Zener diode D, and thep type floating region 55.

The same actions and effects as the actions and effects described abovefor the first preferred embodiment can be exhibited by such anarrangement as well.

Although preferred embodiments of the present invention have beendescribed above, the present invention may also be implemented in yetother modes.

With the first preferred embodiment described above, an example wherethe n⁺ type portions 52 and the p type portions are formed in stripes inthe bidirectional Zener diode D was described. However, thebidirectional Zener diode D may instead have a structure such as shownin FIG. 13.

FIG. 13 is a sectional view of a portion corresponding to FIG. 4 andshows a modification example of the bidirectional Zener diode D. In FIG.13, arrangements that are the same as the arrangements described abovewith the first preferred embodiment are provided with the same referencesymbols and description thereof shall be omitted.

With the bidirectional Zener diode D according to the presentmodification example, an n⁺ type portion 52 at one end portion isdisposed at a center of an inner region of the gate pad 9. The remainingp type portions 53 and n⁺ type portions 52 are disposed concentricallyso as to surround the central n⁺ type portion 52. The first contact plug73 is connected to the n⁺ type portion 52 at the one end portion that ispositioned at the center. Also, the second contact plug 74 is connectedto an n⁺ type portion 52 at another end portion that is positioned at anoutermost periphery.

Even with such a structure, the bidirectional Zener diode D, having thestructure where the n⁺ type portions 52 and the p type portions 53 arerepeated alternately from the n⁺ type portion 52 at one end portion tothe n⁺ type portion 52 at the other end portion, can be obtained.Obviously, the bidirectional Zener diode D of such structure may also beapplied to the second preferred embodiment and the third preferredembodiment.

With the first preferred embodiment described above, an example wherethe front surface electrode 8 includes the gate pad 9, which is formedalong the corner portion of the semiconductor layer 2 was described.However, a front surface electrode 8 having a structure shown in FIG. 14may be adopted instead.

FIG. 14 is a plan view of a modification example of the front surfaceelectrode. In FIG. 14, arrangements that are the same as thearrangements described above with the first preferred embodiment areprovided with the same reference symbols and description thereof shallbe omitted. An example where the semiconductor layer 2 is formed to achip shape of oblong shape in plan view is shown in FIG. 14.

With the front surface electrode 8 according to the present modificationexample, the source pad 11 is formed to an oblong shape in plan viewthat extends along a length direction of the semiconductor substrate 21.The source pad 11 has formed therein a removed region 99, which extendsfrom one end portion toward another end portion of a length direction ofthe source pad 11 and with which one end portion is an open end andanother end portion is a closed end. The closed end of the removedregion 99 is arranged as a pad region 100 that is wider than anothersection of the removed region 99.

The gate pad 9 is disposed in the pad region 100 of the removed region99. With the exclusion of a portion of connection with the gate finger10, the gate pad 9 is surrounded by the source pad 11. The gate finger10 extends from the gate pad 9 toward the open end of the removed region99 of the source pad 11. Further, the gate finger 10 is routed to theouter region 7 (outer periphery of the source pad 11) from the open endof the removed region 99. The gate finger 10 may surround the entireperiphery of the source pad 11.

Even with such a structure, the trench diode structure 45, extendingacross the gate pad 9 and the source pad 11, may be formed in a regiondirectly under the gate pad 9. Obviously, the front surface electrode 8of such structure may also be applied to the second preferred embodimentand the third preferred embodiment.

With each of the preferred embodiments described above, an example wherethe trench gate structures 27, the trench diode structure 45, and theelectric field relaxation structures 61 have substantially the samearrangement was described. However, the trench gate structures 27, thetrench diode structure 45, and the electric field relaxation structures61 may instead have respectively different structures.

For example, the gate trenches 28, the diode trench 46, and the electricfield relaxation trenches 62 maybe formed at respectively differentdepths by forming the gate trenches 28, the diode trench 46, and theelectric field relaxation trenches 62 in separate processes,respectively.

Also, the first inner wall insulating film 29, the second inner wallinsulating film 47, and the third inner wall insulating film 63 may beformed to respectively different thicknesses by forming the first innerwall insulating film 29, the second inner wall insulating film 47, andthe third inner wall insulating film 63 in separate processes,respectively.

Also, at least one of the first inner wall insulating film 29, thesecond inner wall insulating film 47, and the third inner wallinsulating film 63 may be formed to be of uniform thickness.

Also, the first inner wall insulating film 29 and the third inner wallinsulating film 63 may be formed to be of uniform thickness while thesecond inner wall insulating film 47 integrally includes the second sidewall insulating film 48, the second bottom wall insulating film 49, andthe second connection insulating film 50.

Also, the first inner wall insulating film 29 may be formed to be ofuniform thickness while the third inner wall insulating film 63integrally includes the third side wall insulating film 66, the thirdbottom wall insulating film 67, and the third connection insulating film68.

Also with each of the preferred embodiments described above, a structurein which the conductivity types of the respective semiconductor portionsare inverted may be adopted. That is, a p type portion may be of an ntype and an n type portion may be of a p type.

Besides the above, various design modifications may be made within thescope of the matters described in the claims. Examples of featuresextracted from the present specification and drawings are indicatedbelow.

Clause 1: A semiconductor device including a semiconductor layer of afirst conductivity type, having a main surface, a diode trench, formedin the main surface of the semiconductor layer, a bidirectional Zenerdiode, formed inside the diode trench and having a pair of firstconductivity type portions and at least one second conductivity typeportion formed between the pair of first conductivity type portions, anda bottom wall insulating film, which is interposed between thebidirectional Zener diode and a bottom wall of the diode trench and withwhich a ratio of thickness with respect to depth of the diode trench isset to not less than 0.08 and not more than 0.35.

With the semiconductor device according to Clause 1, the bottom wallinsulating film is interposed between the bottom wall of the diodetrench and the bidirectional Zener diode. The ratio of the thickness ofthe bottom wall insulating film with respect to the depth of the diodetrench is set to not less than 0.08 and not more than 0.35. An inversionsuppressing structure, which suppresses inversion of the conductivitytype of the second conductivity type portion of the bidirectional Zenerdiode to the first conductivity type, is formed by the bottom wallinsulating film.

The inversion of the conductivity type of the second conductivity typeportion to the first conductivity type can thereby be suppressed evenwhen a voltage drop occurs across the pair of first conductivity typeportions. A semiconductor device with which undesired increase ofcurrent can be suppressed can thus be provided.

Clause 2: The semiconductor device according to Clause 1, where aninversion suppressing structure, which suppresses inversion of theconductivity type of the second conductivity type portion of thebidirectional Zener diode to the first conductivity type, is formed bythe bottom wall insulating film.

Clause 3: The semiconductor device according to Clause 1 or 2, where thedepth of the diode trench is not less than 9000 Å and not more than12000 Å, and the thickness of the bottom wall insulating film is notless than 1000 Å and not more than 3000 Å.

Clause 4: The semiconductor device according to any one of Clauses 1 to3, further including a side wall insulating film, formed along sidewalls of the diode trench and having a thickness less than the thicknessof the bottom wall insulating film.

Clause 5: The semiconductor device according to Clause 4, where a ratioof the thickness of the side wall insulating film with respect to thethickness of the bottom wall insulating film is not less than 0.16 andnot more than 0.6.

REFERENCE INVENTION

There has been an increasing demand for transistors that meetrequirements of low on-resistance and low capacitance (high speedswitching) in recent years, for example, in the onboard devices market,industrial machinery market, etc. For example, measures are being takento make an area of an active region of a transistor small to reduce aneffective area of each gate electrode and reduce an input capacitanceCiss (=Cgd+Cgs). However, such lowering of capacitance of a transistorbrings about a conflicting effect of lowering of electrostatic breakdownresistance of the transistor.

A countermeasure that is generally taken against electrostatic breakdownis to incorporate a bidirectional Zener diode in a transistor as inPatent Document 1.

However, with an arrangement where the bidirectional Zener diode is madeto undergo avalanche breakdown to release a noise current as in PatentDocument 1, for example, a portion of a p⁻ type region remains at afixed width without becoming depleted even after breakdown. Theremaining portion of the p⁻ type region becomes a serial resistor in theflow of the noise current through the bidirectional Zener diode. Thenoise current is thus not absorbed sufficiently and readily leads tobreakdown of a gate insulating film.

Although on the other hand, a countermeasure of thickening the gateinsulating film to improve a breakdown resistance of the gate insulatingfilm itself maybe considered, it is difficult to adjust a thickness ofthe gate insulating film precisely in accordance with a design value ofthe breakdown resistance of the film. It is also not preferable tochange the thickness of the gate insulating film needlessly because thethickness of the gate insulating film is also closely related to theswitching performance of the transistor.

An object of a reference invention is to provide a semiconductor device,which, although being of low on-resistance and low capacitance, iscapable of realizing a high electrostatic breakdown resistance.

A semiconductor device according to a preferred embodiment of thereference example includes a semiconductor layer, having a source regionof a first conductivity type, a body region of a second conductivitytype, and a drain region of the first conductivity type, a gateelectrode, facing the body region via a gate insulating film, a sourceelectrode, connected to the source region, and a bidirectional Zenerdiode, which has a pair of first conductivity type portions atrespective end portions and at least one second conductivity typeportion between the pair of first conductivity type portions and withwhich the pair of first conductivity type portions are respectivelyconnected to the source electrode and the gate electrode, and the secondconductivity type portion of the bidirectional Zener diode has a widthsmaller than a width of a depletion layer that spreads from a pnjunction portion of the first conductivity type portion and the secondconductivity type portion when a predetermined voltage is applied acrossthe source electrode and the gate electrode.

With the present arrangement, the width of the depletion layer thatforms when the predetermined voltage is applied across the gate and thesource>the width of the second conductivity type portion of thebidirectional Zener diode. Therefore, the first conductivity typeportion at one end portion of the bidirectional Zener diode can be madeconductive with the first conductivity type portion at the other endportion of the bidirectional Zener diode by punch-through when, forexample, a voltage due to electrostatic discharge is applied across thegate and the source. A serial resistance when current flows afterbreakdown can thereby be reduced in comparison to a structure with whichthe bidirectional Zener diode is made conductive by being made toundergo avalanche breakdown. Consequently, even when noise currententers inside the semiconductor device due to electrostatic discharge,the noise current can be released satisfactorily via the bidirectionalZener diode. A high electrostatic breakdown resistance can thereby berealized and lower on-resistance and lower capacitance can thus berealized, for example, by making the respective portions of thesemiconductor device finer.

Also, the gate insulating film does not to have to be made thick toimprove the breakdown resistance and the thickness of the gateinsulating film may be designed with a focus on switching performance ofa transistor. Influences on the switching performance of the transistorcan thus also be kept small.

With the semiconductor device according to the preferred embodiment ofthe reference invention, the “predetermined voltage” may, for example,include a voltage not less than a “rated gate-source voltage Vgss” thatis guaranteed for semiconductors in commercial distribution.

The semiconductor device according to the preferred embodiment of thereference invention may further include a gate trench, formed in thesemiconductor layer, and an embedded insulating film, embedded in abottom portion of the gate trench. In this case, the source region, thebody region, and the drain region maybe aligned in a depth direction ofthe gate trench along side surfaces of the gate insulating film and thegate insulating film may be formed, in continuation to the embeddedinsulating film, on side surfaces of the gate trench. The semiconductordevice may also include a thin film portion that is disposed at aboundary portion between the gate insulating film and the embeddedinsulating film and is thinner than the gate insulating film.

As mentioned above, the semiconductor device according to the preferredembodiment of the reference invention can realize a high electrostaticbreakdown resistance and can therefore be applied satisfactorily to asemiconductor device, having the structure with the thin film portiondisposed inside the gate trench, as well.

With the semiconductor device according to the preferred embodiment ofthe reference invention, the bidirectional Zener diode maybe constitutedof a polysilicon layer and the first conductivity type portions and thesecond conductivity type portion may respectively include firstconductivity type impurity regions and a second conductivity typeimpurity region that are selectively formed in the polysilicon layer.

A withstand voltage (breakdown voltage) of the bidirectional Zener diodeis defined by the width of the second conductivity type portion.Therefore, if the second conductivity type portion is an impurity regioninside the polysilicon layer as mentioned above, the width of the secondconductivity type portion can be adjusted easily and the width of thesecond conductivity type portion can be adjusted with high precision byadjusting a width of a mask used in implanting the correspondingimpurity in the polysilicon layer.

Specifically, if the bidirectional Zener diode is constituted of thepolysilicon layer, the thin film portion may have a thickness of 400 Åto 450 Å, and the second impurity region may have an impurityconcentration of 2.0×10¹⁶ cm⁻³ to 6.0×10¹⁶ cm⁻³ and a width of 2.4 μm to2.6 μm.

With the bidirectional Zener diode in the semiconductor device accordingto the preferred embodiment of the reference invention, the firstconductivity type portion at one end portion may be disposed at a centerand the remaining second conductivity type portion and firstconductivity type portion may be disposed concentrically so as tosurround the first conductivity type portion at the center.

If the semiconductor device according to the preferred embodiment of thereference invention includes a gate pad, which is connected to the gateelectrode and is exposed on a frontmost surface of the semiconductordevice, the bidirectional Zener diode may be disposed in a regiondirectly under the gate pad.

With the present arrangement, by effective use of the region directlyunder the gate pad, it becomes unnecessary to secure an installationspace for the bidirectional Zener diode at an outer peripheral region ofa chip, and therefore contribution can be made toward making thesemiconductor device fine.

With the semiconductor device according to the preferred embodiment ofthe reference invention, the semiconductor layer may include a siliconsubstrate.

A preferred embodiment of the reference invention shall now be describedin detail with reference to the attached drawings.

FIG. 15 is a schematic plan view of a semiconductor device 101 accordingto the preferred embodiment of the reference invention.

With reference to FIG. 15, the semiconductor device 101 includes asemiconductor substrate 102 being an example of the semiconductor layerof the reference invention, an electrode film 103, and a front surfaceprotection film 104. The front surface protection film 104 covers theelectrode film 103 partially and selectively exposes a source pad 110and a gate pad 111 which are to be described below. For example, siliconnitride (SiN), etc., may be used as the front surface protection film104.

The semiconductor substrate 102 defines an outer shape of thesemiconductor device 101 and, for example, has a chip shape of oblongshape in plan view. The semiconductor substrate 102 may, for example, bea silicon substrate and besides this, may be of a wide band gapsemiconductor, such as silicon carbide (SiC), gallium nitride (GaN),etc., that may be used in a power device.

The electrode film 103 is constituted of a conductive material, such asan aluminum (Al) based material (for example, AlCu, etc.), and includesa source metal 105 and a gate metal 106.

The source metal 105 has an outer shape of substantially oblong shape inplan view that extends in a length direction of the semiconductorsubstrate 102. The source metal 105 has formed therein a removed region107, which extends from one end portion toward another end portion ofthe length direction of the source metal 105 and with which one end isan open end and another end is a closed end. The closed end of theremoved region 107 is arranged as a pad region 108 that is wider thananother section of the removed region 107. Also, a portion of the sourcemetal 105 is exposed as a source pad 110 at a frontmost surface of thesemiconductor device 101 from a pad opening 109 in the front surfaceprotection film 104.

The gate metal 106 includes a gate pad 111 and a gate finger 112.

The gate pad 111 is disposed in the pad region 108 and is exposed at thefrontmost surface of the semiconductor device 101 from a pad opening 113in the front surface protection film 104. With the exclusion of aportion of connection with the gate finger 112, the gate pad 111 issurrounded by the source metal 105.

On the other hand, the gate finger 112 (indicated for the sake ofclarity by solid lines in FIG. 15) may be covered by the front surfaceprotection film 104. The gate finger 112 extends from the gate pad 111toward the open end of the removed region 107 of the source metal 105,is further routed from the open end to a peripheral edge portion of thesemiconductor substrate 102, and surrounds the source metal 105. In thepresent preferred embodiment, the entire periphery of the source metal105 is surrounded by the gate finger 112.

FIG. 16 is a sectional view of the semiconductor device 101 taken alongsection line XVI-XVI in FIG. 15. FIG. 17 is a diagram of a principalportion of a gate insulating film 115 of FIG. 16.

With reference to FIG. 16, the semiconductor device 101 includes thesemiconductor substrate 102, gate trenches 114, gate insulating films115, gate electrodes 116, p⁻ type body regions 117, n⁺ type sourceregions 118, an n⁻ type drain region 119, p⁺ type body contact regions120, an interlayer insulating film 121, the source metal 105, and adrain electrode 122.

The semiconductor substrate 102 may, for example, be an epitaxialsubstrate obtained by crystal growth of an epitaxial layer 124,constituted of an n⁻ type silicon, on a base substrate 123 of an n⁺ typesilicon. An impurity concentration of the n⁺ type base substrate 123may, for example, be 2.0×10¹⁹ cm⁻³ to 7.0×10¹⁹ cm⁻³, and an impurityconcentration of the n⁻ type epitaxial layer 124 may, for example, be8.0×10¹⁵ cm⁻³ to 2.0×10¹⁶ cm⁻³.

The gate trenches 114 are formed in a predetermined pattern on a frontsurface portion of the semiconductor substrate 102. The pattern of thegate trenches 114 may be any of various patterns, such as stripes,lattice, etc. The front surface portion of the semiconductor substrate102 is partitioned into a plurality of unit cells 125 in accordance withthe pattern of the gate trenches 114. Also, an interval (cell pitch)between mutually adjacent gate trenches 114 may, for example, beapproximately 1.0 μm to 2.0 μm.

The gate insulating films 115 are formed on inner surfaces of the gatetrenches 114 and shall be described more specifically with reference toFIG. 17.

With reference to FIG. 17, in addition to a gate insulating film 115, anembedded insulating film 126 is disposed in an interior of each gatetrench 114. The gate insulating film 115 and the embedded insulatingfilm 126 may both be constituted of an insulating material, such assilicon oxide (SiO₂), etc. The embedded insulating film 126 is embeddedto a fixed height from a deepest portion of the gate trench 114, and thegate insulating film 115 is disposed, in continuation to the embeddedinsulating film 126, on side surfaces of the gate trench 114. Athickness t₁ of the embedded insulating film 126 in a depth direction ofthe gate trench 114 is, for example, 1000 Å to 3000 Å, and a thicknesst₂ of the gate insulating film 115 is, for example, 500 Å to 600 Å.

A gate electrode 116 is embedded in a region surrounded by the gateinsulating film 115 and the embedded insulating film 126. The gateelectrode 116 may, for example, be constituted of a conductive material,such as polysilicon, etc.

Also with the present preferred embodiment, a thin film portion 127,having a thickness t₃ (for example of 400 Å to 450 Å) that is thinnerthan the thickness t₂ of the gate insulating film 115, is providedintegrally at a boundary portion between the gate insulating film 115and the embedded insulating film 126. That is, with the insulating filmsinside each gate trench 114, the thin film portion 127 is formed at aportion contacting a bottom portion of the gate electrode 116. The thinfilm portion 127 may be constituted of the same insulating material,such as silicon oxide (SiO₂), etc., as the gate insulating film 115 andthe embedded insulating film 126.

Referring again to FIG. 16, in each unit cell 125, a p⁻ type body region117 is formed on a front surface portion of the epitaxial layer 124. Animpurity concentration of the p⁻ type body region 117 may, for example,be 1.0×10¹⁶ cm⁻³ to 3.5×10¹⁶ cm⁻³.

In each unit cell 125, n⁺ type source regions 118 are formed on a frontsurface portion of the p⁻ type body region 117. An impurityconcentration of the n⁺ type body regions 118 may, for example, be1.0×10¹⁹ cm⁻³ to 1.0×10²⁰ cm⁻³.

The n⁻ type drain region 119 is, in the epitaxial layer 124, an n⁻ typeportion at a side of the p⁻ type body region 117 opposite the n⁺ typesource regions 118 and is a region in common to the plurality of unitcells 125. Also, the n⁻ type drain region 119 is a region in which theconductivity type of the epitaxial layer 124 is maintained. An impurityconcentration of the n⁻ type drain region 119 may thus be, for example,2.0×10¹⁶ cm⁻³ to 6.0×10¹⁶ cm⁻³ (same as that of the n⁻ type epitaxiallayer 124). The n⁻ type drain region 119 may be also be referred to asan n⁻ type drift region.

In each unit cell 125, a p⁺ type body contact region 120 is formed topenetrate through the n⁺ type source regions 118 from the front surfaceof the epitaxial layer 124 and contact the p⁻ type body region 117. Animpurity concentration of the p⁺ type body contact region 120 may, forexample, be 1.0×10¹⁶ cm⁻³ to 3.5×10¹⁶ cm⁻³.

The interlayer insulating film 121 is constituted of an insulatingmaterial, such as silicon oxide (SiO₂), etc., and is disposed on thesemiconductor substrate 102. The interlayer insulating film 121 hasformed therein contact holes 128 that expose the n⁺ type source regions118 and the p⁺ type body contact regions 120. The source metal 105 isconnected to the n⁺ type source regions 118 and the p⁺ type body contactregions 120 via the contact holes 128.

The drain electrode 122 is constituted of a conductive material, such asan aluminum (Al) based material (for example, AlCu, etc.), and is formedon a rear surface of the semiconductor substrate 102.

FIG. 18 is an enlarged view of a region surrounded by broken line XVIIIin FIG. 15. FIG. 19 is a sectional view taken along section line XIX-XIXin FIG. 4. In FIG. 18 and FIG. 19, arrangements indicated in FIG. 15 toFIG. 17 and described above are provided with the same symbols anddescription thereof may be omitted.

With reference to FIG. 18 and FIG. 19, a bidirectional Zener diode 129is disposed in a region directly under the gate pad 111. Thebidirectional Zener diode 129 is formed above the semiconductorsubstrate 102 via an insulating film 130 (for example, of silicondioxide). The bidirectional Zener diode 129 has a structure, in which n⁺type portions 131, each being an example of the first conductivity typeportion of the reference invention, and p⁻ type portions 132, each beingan example of the second conductivity type portion of the referenceinvention, are repeated alternately, and respective end portions of therepetition structure are n⁺ type portions 131. As shown in FIG. 18, inthe present preferred embodiment, the bidirectional Zener diode 129 hasthe n⁺ type portion 131 at one end portion disposed at a center of aninner region of the gate pad 111 and has the remaining p⁻ type portions132 and n⁺ type portions 131 disposed concentrically so as to surroundthe central n⁺ type portion 131.

Also with the present preferred embodiment, the bidirectional Zenerdiode 129 is constituted of a polysilicon layer. The n⁺ type portions131 and the p⁻ type portions 132 are arranged as n type or p typeimpurity regions formed selectively in the polysilicon layer. Forexample, the n⁺ type portions 131 and the p⁻ type portions 132 may havethe same impurity concentrations as the n⁺ type source regions 118 andthe p⁻ type body regions 117, respectively.

The interlayer insulating film 121 covers the bidirectional Zener diode129. Gate side contact holes 133, exposing the n⁺ type portion 131 atthe one end portion, are formed in the inner region of the gate pad 111.

Each gate side contact hole 133 is formed, for example, to a rectilinearshape along a peripheral edge of the n⁺ type portion 131 at the center.In the present preferred embodiment, the n⁺ type portion 131 at thecenter is formed to a quadrilateral shape in plan view and a total offour gate side contact holes 133 are formed, one each at each peripheraledge of the n⁺ type portion 131 at the center.

The gate pad 111 is connected to the n⁺ type portion 131 at the centervia contact plugs 134 (conductive material, for example, tungsten (W),etc.) that are embedded in the respective gate side contact holes 133.

Also, in the interlayer insulating film 121, a source side contact hole135, exposing the n⁺ type portion 131 at the other end portion, isformed directly under the source pad 110 at the rear of the gate pad 111(at an opposite side of a position of the gate pad 111 connected to thegate finger 112).

The source side contact hole 135 is formed, for example, to arectilinear shape along one peripheral edge of the n⁺ type portion 131at an outermost periphery. In the present preferred embodiment, the n⁺type portion 131 at the outermost periphery is formed to a quadrilateralannular shape in plan view and the source side contact hole 135 isformed along the one peripheral edge of the n⁺ type portion 131 at theoutermost periphery.

The source pad 110 is connected to the n⁺ type portion 131 at theoutermost periphery via a contact plug 136 (conductive material, forexample, tungsten (W), etc.) that is embedded in the source side contacthole 135.

A method for manufacturing the semiconductor device 101 shall now bedescribed. FIG. 20 is a flow diagram of a method for manufacturing thesemiconductor device 101. FIG. 21A to FIG. 21D are sectional views fordescribing processes related to the forming of the gate insulating film115.

To manufacture the semiconductor device 101, for example, the epitaxiallayer 124, constituted of n⁻ type silicon, is formed by epitaxial growthon the n⁺ type silicon base substrate 123 (S1).

The gate trenches 114 and the gate insulating film 115 are formed in thenext step. As shown in FIG. 21A, the gate trenches 114 are formed in theepitaxial layer 124 by a method, such as reactive ion etching (RIE),etc. (S2).

Next, as shown in FIG. 21B, an insulating film 137 is deposited on theepitaxial layer 124, for example, by the CVD method (S3). The depositionof the insulating film 137 is continued until the gate trenches 114 arerefilled and the front surface of the epitaxial layer 124 is covered bythe insulating material.

Next, as shown in FIG. 21C, a portion of the insulating film 137 isremoved, for example, by etching back (S4). The embedded insulatingfilms 126, constituted of the insulating film 137 remaining at bottomportions of the gate trenches 114, are thereby obtained.

Next, as shown in FIG. 21D, the side surfaces of the gate trenches 114are oxidized, for example by thermal oxidation, to form the gateinsulating films 115 (S5).

Although specific illustration shall be omitted again from here, afterthe forming of the gate insulating films 115, polysilicon, which is thematerial of the gate electrodes 116, is deposited, for example, by theCVD method (S6), and after the deposition, unnecessary portions areremoved, for example, by etching back (S7). The gate electrodes 116,embedded in the gate trenches 114, are thereby obtained.

Next, the insulating film 130, which is to become a base of thebidirectional Zener diode 129, is formed on the epitaxial layer 124, forexample, by the CVD method (S8).

Next, polysilicon, which is the material of the bidirectional Zenerdiode 129, is deposited, for example, by the CVD method (S9), and afterthe deposition, unnecessary portions are removed, for example, byetching back (S10). The polysilicon layer for diode is thereby obtainedon the insulating film 130.

Next, a mask having openings selectively at regions at which the n⁺ typesource regions 118 are to be formed is applied above the epitaxial layer124 and an n type impurity is implanted via the mask. In this process,the n type impurity is also implanted into the polysilicon layer fordiode at the same time (S11). The implantation into the polysiliconlayer for diode may be performed by full surface implantation withoutapplication of a mask.

Next, a mask having openings selectively at regions at which the p⁻ typebody regions 117 are to be formed is applied above the epitaxial layer124 and a p type impurity is implanted via the mask. In this process,the p type impurity is also implanted into the polysilicon layer fordiode at the same time (S12). The implantation into the polysiliconlayer for diode is performed by applying a mask to regions in which thep⁻ type portions 132 are not to be formed.

Next, a mask having openings selectively at regions at which the p⁺ typebody contact regions 120 are to be formed is applied above the epitaxiallayer 124 and the p type impurity is implanted via the mask (S13).

Thereafter, diffusion processing is performed on the impuritiesimplanted in S11 to S13 to form the p⁻ type body regions 117, the n⁺type source regions 118, the p⁺ type body contact regions 120, and then⁺ type portions 131 and p⁻ type portions 132 of the bidirectional Zenerdiode 129.

Next, the interlayer insulating film 121 is formed on the epitaxiallayer 124, for example, by the CVD method (S14) and the respectivecontact holes 128, 133, and 135 are formed in the interlayer insulatingfilm 121 (S15).

After then embedding the contact plugs 134 and 136 in the gate sidecontact holes 133 and the source side contact hole 135, the source metal105 and the gate metal 106 are formed on the interlayer insulating film121 (S16).

Next, the drain electrode 122 is formed on the rear surface of thesemiconductor substrate 102, for example, by the sputtering method. Thesemiconductor device 101 is obtained through the above processes.

The semiconductor device 101 may, for example, be used as a switchingelement. In this case, in a state of applying a drain voltage across thesource metal 105 and the drain electrode 122 (across the source and thedrain) such that the drain side will be positive, a predeterminedvoltage (a voltage not less than a gate threshold voltage) is applied tothe gate metal 106. Channels are thereby formed along the depthdirection of the gate trenches 114 at portions of the p⁻ type bodyregions 117 in vicinities of the gate insulating films 115, and acurrent flows in the depth direction of the gate trenches 114.

With the present semiconductor device 101, the bidirectional Zener diode129 is connected between the gate metal 106 and the source metal 105(between the gate and the source) as shown in FIG. 18 and FIG. 19.Therefore, even if noise current due to electrostatic discharge, etc.,enters into the semiconductor device 101, the noise current can bereleased to the exterior (ground potential) by making it flow withpriority through the bidirectional Zener diode 129.

That is, as shown in FIG. 22, if the breakdown resistance of the gateinsulating films 115 (the gate insulating films 115 here conceptuallyinclude the thin film portions 127) is close to 25V, when a voltage ofapproximately 25V is applied across the gate and the source in a statewhere the bidirectional Zener diode 129 is not incorporated, the gateinsulating films 115 will break down and a leak current will flow near25V. The leak current is more significant the higher the gate-sourcevoltage Vgs and increases substantially proportionately from the voltagevalue corresponding to the breakdown resistance.

On the other hand, as indicated by breakdown waveforms for two diodes inFIG. 22, it is considered that when the bidirectional Zener diode 129that undergoes breakdown at a lower voltage than the breakdownresistance of the gate insulating films 115 is incorporated, even uponentry of noise current, etc., the bidirectional Zener diode 129 can bemade to undergo breakdown before the gate insulating films 115 tothereby make the noise current flow with priority through thebidirectional Zener diode 129 and achieve ESD protection of thesemiconductor device 101.

However, with a diode of a first mode in FIG. 22, the waveform afterbreakdown is considerably gradual in comparison to the waveform for thegate insulating films 115 and therefore from a gate-source voltage Vgsof approximately 40V (at an intersection A of the diode waveform and thegate insulating film waveform) as a boundary, the noise current flowsout into the gate insulating films 115, so that the gate insulatingfilms 115 undergo breakdown.

On the other hand, when, as with a diode of a second mode in FIG. 22,the waveform after breakdown is steep in comparison to that of the firstmode and a slope thereof is close to a slope of the waveform for thegate insulating films 115, even if a comparatively high gate-sourcevoltage Vgs is applied, the noise current can be made to continue toflow through the bidirectional Zener diode 129 with priority, so thatbreakdown of the gate insulating films 115 can be prevented.

Mechanisms of breakdown of the diode of the first mode and the diode ofthe second mode shall now be described in detail with reference to FIG.23 and FIG. 24.

First, FIG. 23 is a diagram for describing the mechanism of breakdown ofthe bidirectional Zener diode 129 according to the first mode, withwhich the bidirectional Zener diode 129 undergoes breakdown by avalanchebreakdown (avalanche design).

As shown in FIG. 23, the withstand voltage (breakdown voltage) of thebidirectional Zener diode 129 is defined by the impurity concentrationsof the n⁺ type portions 131 and the p⁻ type portions 132 and thereforewhen the gate-source voltage Vgs corresponding to the breakdown voltageis applied, the n⁺ type portion 131 at the source side and the n⁺ typeportion 131 at the gate side are made conductive to each other due toavalanche breakdown. Therefore, even after breakdown, each p⁻ typeportion 132 is not filled up by a depletion layer 138 (lower side ofFIG. 23) and portion of the each p⁻ type portion 132 remains at a fixedwidth. The remaining portions of the p⁻ type portions 132 become serialresistors 139 in the flow of noise current through the bidirectionalZener diode 129 from the source side to the gate side. Therefore asshown in FIG. 22 and FIG. 25, the noise current (ordinate axis lgs inFIG. 22 and FIG. 25) increases only gradually even after breakdown andreadily leads to breakdown of the gate insulating films 115.

Meanwhile, FIG. 24 is a diagram for describing the mechanism ofbreakdown of the bidirectional Zener diode 129 according to the secondmode, with which the bidirectional Zener diode 129 undergoes breakdownby punch-through (punch-through design).

As shown in FIG. 24, the withstand voltage (breakdown voltage) of thebidirectional Zener diode 129 is defined by a width of each p⁻ typeportion 132. Specifically, the width Wp of each p⁻ type portion 132 is,for example, defined to be a width smaller than a width Wd of eachdepletion layer 138 that spreads when the breakdown voltage is appliedacross the gate and the source (Wp≤Wd). Therefore, when a gate-sourcevoltage Vgs, corresponding to the breakdown voltage of the bidirectionalZener diode 129, is applied, the n⁺ type portion 131 at the source sideand the n⁺ type portion 131 at the gate side can be made conductive toeach other by punch-through. Therefore, after breakdown, the region ofeach p⁻ type portion 132 is filled up with the depletion layer 138(lower side of FIG. 24) and therefore the serial resistance can bereduced in comparison to the avalanche design of FIG. 23. Thereby asshown in FIG. 22 and FIG. 25, the noise current (ordinate axis lgs inFIG. 22 and FIG. 25) after breakdown is made to increase steeply incomparison to the avalanche design of FIG. 23 and the noise current canthus be released satisfactorily to the ground potential via thebidirectional Zener diode 129.

FIG. 26 is a diagram for describing how a gate-source breakdown voltageBVgss and the electrostatic breakdown resistance vary according to adesign dimension of the p⁻ type portion 132 of the bidirectional Zenerdiode 129.

As shown in FIG. 26, the gate-source breakdown voltage BVgss convergesat approximately 27V at a design dimension of 2.6 μm as a boundary andremains there even when the width of each p⁻ type portion 132 increasesfurther. On the other hand, the electrostatic breakdown resistancedecreases rapidly in a region of 2.6 μm and greater. That is, in aregion where the width of the p⁻ type portion 132 reaches 2.6 μm as aboundary and increases further, breakdown due to avalanche breakdown,which is defined by the concentration of the p⁻ type portion 132, occursregardless of the width of the p⁻ type portion 132 and consequently,breakdown of the gate insulating films 115 occurs readily (theelectrostatic breakdown resistance is low).

On the other hand, in a region where the width of the p⁻ type portion132 is not more than 2.6 μm, the gate-source breakdown voltage BVgssincreases in proportion to the design dimension of the p⁻ type portion132 and it can thus be confirmed that breakdown due to punch-through,which is related to the width of the p⁻ type portion 132, is occurring.Therefore, in the region where the width of the p⁻ type portion 132 isnot more than 2.6 μm, noise current is absorbed satisfactorily inaccordance with the mechanism shown in FIG. 24 and therefore a highelectrostatic breakdown resistance is maintained.

If, for example, the “rated gate-source voltage Vgss” guaranteed for thesemiconductor device 101 is 20V, application of a gate-source voltageexceeding the rated voltage Vgss will be a burden on the gate insulatingfilms 115 and therefore, based on FIG. 26 and allowing for a variationof 0.1 μm, the design dimension of the p⁻ type portion 132 is set to 2.5μm (±0.1 μm). By this arrangement, when 20V is exceeded, thebidirectional Zener diode 129 can be made to undergo breakdown bypunch-through to sufficiently release noise current and meanwhilesuppress current flowing in the gate insulating films 115. Consequently,a high electrostatic breakdown resistance of approximately 30V can bemaintained.

The preferable design dimension described above is merely an example forverifying the effects of the semiconductor device 101 and variesaccording to the impurity concentration of the p⁻ type portions 132 thatconstitute the bidirectional Zener diode 129. For example, if theimpurity concentration of the p⁻ type portions 132 is in a range of,2.0×10¹⁶ cm⁻³ to 6.0×10¹⁶ cm⁻³, the width is preferably 2.4 μm to 2.6μm.

As described above, a high electrostatic breakdown resistance can berealized by the semiconductor device 101.

This can be demonstrated further by way of FIG. 27. FIG. 27 is a diagramthat shows that even when an active region of the semiconductor device101 is made small in area and fine, a sufficient electrostaticresistance can be realized. The second mode and the first mode in FIG.27 correspond to the second mode and the first mode shown in FIG. 22 toFIG. 25 and described above. On the other hand, the conventional exampleindicates a semiconductor device having the same arrangement as that ofthe first mode with the exception that, in each gate trench 114, theembedded insulating film 126 and the thin film portion 127 are notformed and a gate insulating film thicker than the thin film portion 127is formed to a substantially uniform thickness.

That is, as shown in FIG. 27, with the semiconductor device of thesecond mode, a high electrostatic breakdown resistance is realized forthe semiconductor device as a whole despite the breakdown resistance ofthe insulating films inside the gate trenches 114 themselves being lowerthan that of the conventional example due to having the thin filmportions 127.

Lower on-resistance and lower capacitance can thus be realized, forexample, by adopting the embedded insulating films 126 that are thick incomparison to the gate insulating films 115 to lower the gate-draincapacitance and making the respective portions of the semiconductordevice 101 finer.

Also, there is no need to make the gate insulating films 115 thick toimprove the breakdown resistance, and therefore the thickness of thegate insulating films 115 can be designed with a focus on the switchingperformance of the transistor. Influences on the switching performanceof the transistor can thus also be kept small.

Although a preferred embodiment of the reference invention has beendescribed above, the reference invention may also be implemented in yetother modes.

For example, an arrangement in which the conductivity types of therespective semiconductor portions of the semiconductor device 101 areinverted may be adopted. That is, in the semiconductor device 101, a ptype portion may be of an n type and an n type portion may be of a ptype.

1-18. (canceled)
 19. A semiconductor device comprising: a semiconductorlayer of a first conductivity type, having a main surface; a diodetrench formed in the main surface of the semiconductor layer; an innerwall insulating film formed on an inner surface of the diode trench; abody region of the second conductivity type, formed on the main surfaceof the semiconductor layer; a source region of the first conductivitytype, formed on a surface layer portion of the body region, a drainlayer of the first conductivity type, formed below the body region; agate trench formed in the main surface of the semiconductor layer suchthat the gate trench penetrates the source region and the body regionfrom the main surface of the semiconductor layer to the drain region; agate insulating film formed on an inner surface of the gate trench; anda gate electrode embedded in the gate trench across the gate insulatingfilm, wherein the gate trench has a first inclined portion, a secondinclined portion and a third inclined portion formed from the mainsurface of the semiconductor layer in this order, and an inclinationangle of the second inclined portion with respect to a first directionperpendicular to the main surface of the semiconductor layer is largerthan both of an inclination angle of first inclined portion and aninclination angle of third inclined portion with respect to the firstdirection.
 20. The semiconductor device according to claim 19, wherein,the diode trench has a fourth inclined portion, a fifth inclined portionand a sixth inclined portion formed from the main surface of thesemiconductor layer in this order, and an inclination angle of thefourth inclined portion with respect to the first direction is largerthan both of an inclination angle of fifth inclined portion and aninclination angle of sixth inclined portion with respect to the firstdirection.
 21. The semiconductor device according to claim 19, wherein athickness of a part of the gate insulating film on the second inclinedportion is thinner than a thickness of a part of the gate insulatingfilm on the first inclined portion.
 22. The semiconductor deviceaccording to claim 20, wherein a thickness of a part of the inner wallinsulating film on the fifth inclined portion is thinner than athickness of a part of the gate insulating film on the fourth inclinedportion.
 23. The semiconductor device according to claim 19, wherein abottom portion of the gate electrode is placed at a deeper level thanthe first inclined portion.
 24. The semiconductor device according toclaim 19, wherein the inner wall insulating film includes a side wallinsulating film, formed along side walls of the diode trench, and abottom wall insulating film, formed along a bottom wall of the diodetrench and having a thickness greater than a thickness of the side wallinsulating film.
 25. The semiconductor device according to claim 24,further comprising a bidirectional Zener diode, formed on the inner wallinsulating film and having a pair of first conductivity type portionsand at least one second conductivity type portion formed between thepair of first conductivity type portions, wherein the bottom wallinsulating film includes an inversion suppressing structure that isinterposed between the bottom wall of the diode trench and thebidirectional Zener diode and suppresses inversion of the conductivitytype of the second conductivity type portion of the bidirectional Zenerdiode to the first conductivity type.
 26. The semiconductor deviceaccording to claim 19, further comprising a floating region of thesecond conductivity type formed in a region of the semiconductor layeroriented along a bottom wall of the diode trench.
 27. The semiconductordevice according to claim 19, further comprising a bidirectional Zenerdiode, formed on the inner wall insulating film and having a pair offirst conductivity type portions and at least one second conductivitytype portion formed between the pair of first conductivity typeportions, wherein the bidirectional Zener diode has an upper surfacefacing an opening of the diode trench, and the upper surface of thebidirectional Zener diode is formed on the same plane as the mainsurface of the semiconductor layer.
 28. The semiconductor deviceaccording to claim 19, wherein the bidirectional Zener diode is formedinside the diode trench across an interval from the side walls of thediode trench.
 29. The semiconductor device according to claim 19,further comprising a bidirectional Zener diode, formed on the inner wallinsulating film and having a pair of first conductivity type portionsand at least one second conductivity type portion formed between thepair of first conductivity type portions, wherein the bidirectionalZener diode is formed inside the diode trench across an interval fromthe side walls of the diode trench, and a distance between a side wallof the bidirectional Zener diode and a side wall of the diode trench isgreater than a thickness of the bidirectional Zener diode.
 30. Thesemiconductor device according to claim 28, further comprising a sidewall protection film with insulating property that protects side wallsof the bidirectional Zener diode.
 31. The semiconductor device accordingto claim 19, wherein the bidirectional Zener diode includes apolysilicon body, the pair of first conductivity type portions includefirst conductivity type impurity regions formed selectively in thepolysilicon body, and the second conductivity type portion includes asecond conductivity type impurity region formed selectively in thepolysilicon body.
 32. The semiconductor device according to claim 19,wherein the gate trench has the same depth as a depth of the diodetrench, the gate insulating film has the same structure as the innerwall insulating film, and the gate electrode has the same conductivematerial as the bidirectional Zener diode.
 33. The semiconductor deviceaccording to claim 19, further comprising an electric field relaxationstructure, formed in a surface layer portion of the main surface of thesemiconductor layer in a peripheral region along a peripheral edge ofthe diode trench and relaxing an electric field in the peripheralregion.
 34. The semiconductor device according to claim 33, wherein aplurality of the electric field relaxation structures are formed atintervals in a direction away from the diode trench.
 35. Thesemiconductor device according to claim 33, wherein the electric fieldrelaxation structure is formed so as to surround the diode trench. 36.The semiconductor device according to claim 33, wherein an electricfield relaxation trench is further formed in the main surface of thesemiconductor layer and the electric field relaxation structure includesan electric field relaxation inner wall insulating film, formed along aninner wall of the electric field relaxation trench, and an embeddedconductor, embedded in the electric field relaxation trench across theelectric field relaxation inner wall insulating film.
 37. Thesemiconductor device according to claim 36, wherein the electric fieldrelaxation trench has the same depth as a depth of the diode trench, theelectric field relaxation inner wall insulating film has the samestructure as the inner wall insulating film, and the embedded conductorhas the same conductive material as the bidirectional Zener diode. 38.The semiconductor device according to claim 19, further comprising abidirectional Zener diode, formed on the inner wall insulating film andhaving a pair of first conductivity type portions and at least onesecond conductivity type portion formed between the pair of firstconductivity type portions.